A diaphragm is usually fitted between an x-ray source and the object under examination in x-ray imaging systems, especially in computed tomography systems, which initially defines the aperture of an x-ray bundle and the shape of a surface illuminated by the x-rays. A radiation shaping filter is frequently disposed after this diaphragm in the beam path of the x-rays, which can additionally change the x-rays in their intensity spatially or also spectrally. This generally involves planar filters which the entire x-ray beam (typically delimited by the diaphragm) passes through, without the filter in such cases having to have openings through which the x-rays could pass unchanged. Typically these filters are constructed of aluminum or Teflon.
For manipulation and further modification of the spectral or also spatial intensity distribution of the x-rays, different types of radiation shaping filters, such as so-called wedge filters or also bowtie filters (i.e. filters which additionally focus or widen out the x-rays with convex or concave-shaped surfaces, typically similar to the shape of a bowtie) are used, which can be inserted individually or in a combination of a number of radiation shaping filters into the beam path of the x-rays between an x-ray source and an object under examination. The intensity of x-rays can for example be reduced with the aid of a wedge filter transverse to the direction of propagation of the x-rays by a continuous attenuation value. The intensity minimum in such cases usually lies at the edge of an x-ray bundle used (delimited by the diaphragm). With the aid of a different filter type, such as the bowtie filter mentioned, at least local extremes of the x-ray intensity can be defined within the x-ray bundle for example.
There is also the possibility of defining the size or extent of the irradiated area or the areas of one or more radiation extremes. I.e. along with different types of radiation shaping filters there is also the possibility of selecting between different radiation shaping filters of the same type. For example, with filters of the same type, a choice can be made between “narrow filters” which spatially reduce the irradiated area, or “wide filters” and “very wide” filters, which might possibly expand the irradiated area or the area of an intensity extreme.
In addition it is likewise conceivable for the radiation shaping filter to especially influence the spectrum of the x-rays used (i.e. the spectral intensity distribution of the x-rays changes during its passage through the filter). For example in a spatial area determined by the filter the spectrum of the x-rays can be hardened, i.e. an intensity maximum of the x-rays is changed to smaller wavelengths. Likewise the spectrum of the x-rays might possibly be set softer with the aid of the filter in the predetermined spatial area (i.e. an intensity maximum is changed towards larger wavelengths).
The operator of an x-ray imaging system charged with taking an x-ray thus has the choice between a plurality of filters and their combination, in order to optimize the process of taking the x-ray. On the one hand the optimization can consist of guaranteeing the image quality of an image provided and on the other hand also of keeping the radiation load imposed on an object under examination when obtaining the image as low as possible. A corresponding optimization is largely based in such cases on the experience of the operator.
In the optimum case, for each use of the imaging system in respect of these optimization targets a suitable scan or examination protocol (i.e. a sequence of control steps) is stored, on the basis of which the imaging in the imaging system is controlled and which may possibly predetermine a radiation shaping filter to be used. If a scan protocol is not available for the application concerned, this must first be created based on the specialized knowledge of the operator. An optimum choice of radiation shaping filter in such cases may possibly not be guaranteed. Furthermore the assignment of radiation shaping filters to specific protocols is complex and hampers a simplification of the operation of an x-ray imaging system.